The invention concerns circuitry for suppressing audible noise, especially noise that occurs when audio equipment is turned on and off.
What is called a mute circuit is usually built into such equipment to supress the noise, which is expressed for example in the form of an unpleasant rattling perceived when the equipment is turned on or off. Audible noise also occurs for example during switching from one receiving channel to another because the new tuning voltage, as it becomes established at a specific time constant, makes one transmitter after another perceptible in accordance with the range of tuning voltages being traversed. The mute circuit also suppresses interference that, in the capacity of intermediate-transmitter noise, occasions unpleasant audible noise.
Positioning a relay or electronic switch at the output terminal of the amplifier to either disconnect or short-circuit it is known. Using relays yields good technical results, but the components are very expensive, and the circuit will eventually no longer operate unobjectionably due to contamination of the relay contacts. Switching transistors, although cheaper, also have certain drawbacks. A decrease of 80 dB requires at least two transistors grounded by way of longitudinal resistors. Since the resistors are rated at at least 300.OMEGA., the output resistance of the amplifier is disadvantageously increased to at least 500.OMEGA.. To keep the distortion factor low at output voltages of up to 2 V, the transistors must have a negative bias of 4 to 5 V, in which case higher output voltages cannot be attained. A mute decreased of more than 80 dB is impossible. Finally, powerful distortions occur during the switch from mute to playback operation.
Positioning two bipolar electronic switches each in series with the load resistor of an amplifier and in its negative-feedback loop is known. In this case the amplifier will have a specific amplification level of 40 dB for example both in its amplification and in its mute mode. The non-linearity of the electronic switch in series with the load impedance result in conjunction with its final resistor in non-linear distortion at the resistor that cannot be completely eliminated by the feedback. A very complicated control circuit (European Pat. No. 0 095 774) that is only technically feasible in integrated circuitry because it is not profitable at too discrete an expenditure is needed to activate the electronic switch.